1. Field of the Invention
The invention in general relates to systems which utilize light scattering principles to detect and count undesirable single particles in fluids, referred to in the art as light scattering particle counters, and more particular to such a particle counter that utilizes a laser diode light source.
2. Statement of the Problem
Particle counters are used to detect and measure the size of individual particles suspended in a fluid. Each particle that is detected is counted, and an indication of the number of particle counts within a channel, with each channel corresponding to a particular size range, is provided. For particle counters to operate effectively, the density of particles in the fluid must be very small—indeed, the particles are generally considered to be contaminants. It is important to distinguish the science of particle counting from other scientific fields, such as photometry and cytometry, which also utilize scattered light, but in which the density of the particles in the fluid is relatively large; often it is the particles of the fluid itself that are detected and analyzed. These latter systems rely on collecting scattered light from thousands, millions, and even billions of particles; therefore, their principles of operation are very different from the principles used in particle counters.
Particle counters are generally used to detect contaminants in extremely pure fluids, such as those used in high tech electronics and the pharmaceutical industry. Generally, small samples of the fluids used in the manufacturing processes are diverted to the particle counters, which sound an alarm if the number and/or size of the particles detected is above a predetermined threshold. Since a small sample of the manufacturing fluid is generally not completely representative of the entire volume of the manufacturing fluid, statistics is used to extrapolate the state of the manufacturing fluid from the sample. The larger the sample, the more representative it is, and the more quickly an accurate determination of the number and size of particles in the manufacturing fluid can be made. It is desirable for a particle counter to detect particles as small as possible, as fast as possible, in as large a sample as possible.
Physical constraints require tradeoffs between the above goals. For example, sample volume and speed usually must be sacrificed to detect smaller particles. This is a direct result of the fact that, for particles to be detected in a particular fluid, the fluid must be constrained to flow through the monitoring region of a particle counter. Physical objects, such as nozzles and flow tubes, must be used to direct the fluid flow to the particle counter monitoring region. If it is desired to detect the particles in the entire sample flow, then scattered light from the entire sample flow must be collected. This generally results in light scattered from the physical constraining objects, such as a nozzle or flow tube, also being collected, which light creates noise in the output. The noise prevents detection of extremely small particles. This noise can be avoided by detecting particles in only a small portion of the sample flow. Particle counters that attempt to count all the particles in a fluid sample are generally referred to as volumetric particle counters, and particle counters that detect particles in only a small portion of the fluid flow are generally referred to as in-situ particle counters.
The word in-situ in Latin literally means in the natural state. That is, ideally, it refers to measurements unaffected by the measurement instrumentation. In an in-situ system, to be unaffected from the constraining elements, the detected particles must be far from the constraining elements, and only particles in a small fraction of the sample fluid flow are detected. In-situ systems commonly process 5% or less of the sampled fluid. As a result of measuring only a selected fraction of fluid flow, however, in-situ systems take more time to achieve a statistically significant determination of the fluid cleanliness level or fluid quality. When measuring particle contamination levels in a clean room environment, this extended measurement time generally incurs the risk that an unacceptably high level of airborne or liquid particle concentration could go undetected for substantial time periods, thereby allowing a large number of manufactured parts to be produced under unacceptably “dirty” conditions. This situation can lead to substantial economic loss owing to the waste of time and production materials in the affected facility.
Since it is practically impossible to actually measure 100% of the particles carried by flowing fluid, herein the term “volumetric” generally corresponds to systems which measure 90% or more of the particles flowing through a measurement device. Volumetric particle measurement systems generally provide the advantage of measuring a greater volume of fluid, whether liquid or gas, within a fixed time period, thereby enabling a more rapid determination of a statistically significant measure of fluid quality. In the case where the particle concentration exceeds a predetermined permissible limit, this more rapid fluid processing generally enables a defective manufacturing process to be halted more quickly and more economically than would be possible employing in-situ measurement systems. However, as indicated above, volumetric measurement systems generally experience more noise than do in-situ systems because the efforts expended to control the location and flow characteristics of the fluid being analyzed generally perturbs the characteristics being measured to a greater extent than does in-situ measurement.
In various circumstances, there may be measurement processes having characteristics which are intermediate between in-situ and volumetric processes. Thus, where in-situ measurement generally corresponds to particle measurement within 5% or less of fluid transported through a measurement device, and volumetric measurement generally corresponds to analysis of 90% or more of such fluid, it will be recognized that measurement processes may be configured to process 10%, 30%, 50%, or other percentages in between the levels associated with in-situ and volumetric operation. Accordingly, herein, the term “non-in-situ” measurement generally corresponds to measurement of a proportion of fluid equal to more than 5% of total fluid flow.
In the field of particle counting, the use of high power illumination generally enhances particle detection. Specifically, higher power levels generally enable the detection of smaller particles than lower power systems. Higher power levels also generally permit particles of a given size to be detected more quickly. Thus, high power lasers are generally used as the light source in particle counters.
Diode lasers have recently been incorporated into particle counters because of their relatively small size, economy and reliability. However diode particle counters have an inherent limitation for use in particle counters that limits their power. The power of diode lasers increases with the size of radiating surface. However, as the size of the radiating surface increases transverse radiation modes increase. FIG. 1 shows the radiation patterns, looking into the laser, for various transfers modes. As can be seen, the radiation in the fundamental transverse electromagnetic mode, designated as TEM00, is compact and concentrated in the center. In the other modes a significant portion of the light is contained in zones separated from and some distance from the center of the beam. In certain applications, such as fiber optics, these features of the non TEM00 modes are not a problem because multiple reflections from the sides of the fiber contains the radiation in a compact space. However, in particle counters, these modes scatter and reflect from the parts of the system constraining the fluid flow and create noise which interferes with the detection of particles and places a lower limit on the size of the particles that can be detected. Thus, particle counters that use laser diodes generally limit the mode to the TEM00 mode, which however limits the amount of power of the diode, because, as indicted above, higher power requires a larger radiating surface, which inherently creates non TEM00 modes.
The problems with using high power diodes are particularly acute in particle counters that detect single particles in liquids, referred to herein as “liquid particle counters”. While gases will remain collimated in constraint-free jets for at least a distance necessary to pass the jets through a laser beam, liquids resist such collimation. Thus, in particle counters, liquids must be constrained by the physical walls of flow cells, and the laser beam must thus pass through the flow cell. The non TEM00 transverse modes scatter and reflect from the flow cell walls creating noise. In addition, bubbles in the fluid, which are often present at start-up, diffract the light from all the modes. If lasers having a power of one watt or greater are used in a liquid particle counter, the heat from the combination of the non TEM00 mode scattering and the diffraction from a bubble will damage the flow cell. Thus, all known commercial liquid particle counters that utilize laser diodes to detect and measure single particles in fluids have, up until now, been limited to single mode systems, typically the TEM00 mode, and thus limited to less than 1 watt in power.
Accordingly, there is a need in the art for a particle counter system and method, particularly a liquid particle counter system and method, which provides high power illumination in a low noise environment and which produces a scattered light energy spectrum which is readily convertible into particle measurement data. Further, to accomplish this in a non-in-situ system would be highly advantageous.